Grana, chloroplast

Many DPO preparations from higher plant sources, such as fruits and leaves, are particulate in nature, because, in these systems, the enzyme appears to be bound to the membranes of the chloroplast grana or to cell wall components. The 02 electrode assay is especially advantageous for such preparations, since their turbidity could interfere with spectrophotometric assays. 

Uribe, E, G. FEBS Letters 36 (2), 143 (1973). ATP synthesis driven by a K+-valinomycin-induced charge imbalance across chloroplast grana membranes... 

As mentioned above, the leaves of all green plants contain the same major carotenoids -carotene (1), lutein (2), viola-xanthin (3), and neoxanthin (4). Smaller amounts of other compounds are often present. These compounds are located in the chloroplast grana as chromoproteins (Britton, 1983 Britton et al., 1982). 

XXVII), and antheroxanthin (XXVIII) are often found. These pigments are localized in the chloroplasts, probably in the chloroplast grana, and they may be present in the form of chromoproteins (Bailey et al., 1966). There is little qualitative difference in the pigments found in chloroplast tissue, but there is considerable quantitative variation (Goodwin, 1976). 

The phenol oxidase (catechol Oxidase) of tea leaves provides a classic example of artifactual organelles, and probably of artifactual substrate specificity as well, both produced by protein-phenolic complexing. Li and Bonnerreported that the enzyme is insoluble, tightly bound to the chloroplast grana. Sanderson demonstrated that conventional extracts from tea leaves contained no soluble protein, and that the "chloroplast" fraction consisted largely of protein-tannin complexes. Addition of hydrated nylon as a phenol adsorbent solubilized the enzyme completely. 

Berg S, Dodge S, Krogmann DW and Dilley RA (197 ) Chloroplast Grana Membrane Carboxyl Groups Their Involvement in Membrane Association, Plant Physiol, 53 619-627. 

It is helpful to think of the photosynthesis reaction as the sum of an oxidation half reaction and a reduction half reaction as shown in Figure 1. In fact, nature does separate these half reactions, in that the reduction of C02 to carbohydrates occurs in the stroma of the chloroplast, the organelle in the leaf where the photosynthesis reaction occurs, - whereas, the light-driven oxidation half reaction takes place on the thylakoid membranes which make up the grana stacks within the chloroplast. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) carries the reducing power and most of the energy to the stroma to drive the fixation of C02 with the help of some additional energy provided... 

Chloroplasts (29-36) are the sites of photosynthesis and their ribosomes can carry out protein synthesis. Chloroplasts that contain chlorophylls and carotenoids, are disc shaped and 4-6 pm in diameter. These plastids are comprised of a ground substance (stroma) and are traversed by thylakoids (flattened membranous sacs). The thylakoids are stacked as grana. In addition, the chloroplasts of green algae and plants contain starch grains, small lipid oil droplets, and DNA. 

Figure 5. A portion of a chloroplast with several crystalloids. Note a presence of crystalloids in association with the envelope as well as around and between the grana (x51,060).

FIGURE 19-38 Chloroplast. (a) Schematic diagram, (b) Electron micrograph at high magnification showing grana, stacks of thylakoid membranes. 

In photosynthetic eukaryotic cells, both the light-de-pendent and the carbon-assimilation reactions take place in the chloroplasts (Fig. 19-38), membrane-bounded intracellular organelles that are variable in shape and generally a few micrometers in diameter. Like mitochondria, they are surrounded by two membranes, an outer membrane that is permeable to small molecules and ions, and an inner membrane that encloses the internal compartment. This compartment contains many flattened, membrane-surrounded vesicles or sacs, the thylakoids, usually arranged in stacks called grana (Fig. 19-38b). Embedded in the thylakoid membranes (commonly called lamellae) are the photosynthetic pigments and the enzyme complexes that carry out the light reactions and ATP synthesis. The stroma (the aqueous phase enclosed by the inner membrane) contains most of the enzymes required for the carbon-assimilation reactions. 

Chloroplasts fill most of the cytoplasm around the junction of three cells of Arabidopsis thaliana seen in this micrograph. Both grana stacks and stroma lamellae (pictured in more detail in Fig. 23-19) can be seen. Also present are several small mitochondria. Portions of the large vacuoles, characteristic of plant cells, are seen at top, right, and bottom. Micrograph courtesy of Kenneth Moore. 

Figure 23-19 (A) Electron micrograph of alfalfa leaf chloroplast. Courtesy of Harry T. Homer, Jr., Iowa State University. (B) Schematic drawing of a chloroplast. From Hall and Rao227 (C) Enlargement of a portion of (A) to show grana stacks more clearly.

ATP synthase, whose knobs also protrude into the stroma. A photosynthetic unit can also be defined chemically by the number of various types of molecules present in a chloroplast membrane for each four manganese atoms (Table 23-2). Separate units contain PSI and PSII. These reaction centers appear to have a different distribution within the thylakoids, the PSI units being located principally in the unstacked membranes and the PSII units in the grana stacks.255 259... 

The photosynthesis machinery in the chloroplasts is in a system of thylakoid membranes, which are stacked in arrays called grana. The grana look like a stack of coins. The thylakoid membranes contain enzymes and protein assemblies that contain the chlorophyll. The two kinds of protein assemblies are called photosystem I and photosystem II. These assemblies absorb energy from light. 

Granum (pi. grana) A closely packed stack of thylakoid membranes in a chloroplast. 

Chloroplasts in plant cells are surrounded by a double membrane and have an internal membrane system of thylakoid vesicles that are stacked up to form grana. The thylakoid vesicles contain chlorophyll and are the site of photosynthesis. Carbon dioxide (C02) fixation takes place in the stroma, the soluble matter around the thylakoid vesicles. 

Thylakoid membrane internal membrane system of chloroplasts folded repeatedly into a stack of disks called grana. 

Chloroplasts of higher plants are saucer-shaped, and from 4 to 10 ym in diameter and 1 to 3 ym thick. The chlorophyll is concentrated in bodies within the chloroplasts called grana, which are about 0.4 ym in diameter. Under the electron microscope, the grana appear as highly organized, precisely stacked lamellae, to which the chlorophyll is bound, imbedded in a stroma matrix. The light and associated electron transport reactions take place in the lamellae, whereas enzymes involved in carbon dioxide fixation are located in the stroma. 

Figure 4 shows a typical ultrastructure of hairy root cells observed under an electron microscope. In the case of photomixotrophic hairy roots, a plastid with a chloroplast-like structure was observed in the light-grown root cells, being associated with thylakoid membranes and grana stacks the particles in the vi-... 

Fig. 4A-C. Electron micrographs of thin sections of pak-bung hairy roots A photomixotro-phic hairy roots obtained from a 13 days culture in light at 7=11.1 W nr2) B, C photo-autotrophic hairy roots cultivated in the sucrose-free medium with 3.0% C02-enriched air supply for 30 days using conical flasks illuminated at 7= 11 W m-2 and shaken at 100 rpm. The abbreviations of 1-3 indicates a chloroplast-like structure with thylakoid membranes and grana stacks (1), chloroplasts (2) and cell walls (3), respectively...

FIGURE 1. Thin section of part of an isolated chloroplast showing the internal thylakoid membrane system which consists of appressed grana lamellae (g) and non-appressed stroma lamellae (s) embedded in the stroma protein matrix and surrounded by a double membrane envelope (e). 

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